Drawing die and method of making the same



Nov. 23; 1954 V. J. P. O'BRIEN DRAWING DIE AND METHOD OF MAKING THE SAME Original Filed Nov. 29, 1947 INVENTOR JOHN F! QBRIEN.

p ATTORNEY United States Patent DRAWING DIE AND METHOD OF MAKING THE SAME John P. OBrien, Turtle Creek, Pa.

Original application November 29, 1947, Serial No. 788,835. Divided and this application July 13, 1953, Serial No. 367,516

2 Claims. (Cl. 76-107) The present invention relates to a drawing die, more particularly to one that is suitable for drawing large quantities of metal, such as copper, to very close tolerances and to the method of making such die. The present application is a division of my copending application Serial No. 788,835, filed November 29, 1947.

It is common .in the art to use either solid or split steel dies for drawing soft metals such as copper. It is also common to use a solid steel die having a hole therethrough into which an insert or nib or material harder than steel is "sweated for wire drawing and the like. Such insert when made of a hard material, such as tungsten carbide, is sintered close to final shape before it is placed into "the holder. After it is so placed and rigidly adhered to the holder by sweating or the like, the die hole is honed by hand with diamond dust or the like inorder to give it the final desired shape and finish. However, such hand shaping or honing is laborious and time-consuming, oftentimes requiring many days of tedious Work and achieving, at most, only a fair degree of accuracy in the final dimensions. Moreover, in view of the somewhat porous nature of the skin or surface of such 'sintered tungsten carbide die, there is a tendency for cracks to develop on the surface as the result of honing and for small portions to chip off. Furthermore, since such surface is of limited hardness, there is a tendency for the die to wear oversize or to become out-of-round or out-ofshape after a limited number of drawings of metal therethrough. As the result thereof, there is a considerable amount of undesirable down time, that is, time during which the machine is inoperable for allowing replacement of dies and the like. This, of course, interrupts production and increases manufacturing costs.

A further disadvantage of hand honing of dies is that due to inaccuracies introduced by the human element, in the formation or the die hole, for example, unequalness of the approach angles on the various sides of the die cavity, there is a great tendency for the metal to become twisted while being drawn. Oftentimes, particularly in drawing longer lengths of metal, the twist is so great as to make the drawn metal unsuitable.

An object of the present invention is to provide a novel die structure which is devoid of the above-mentioned disadvantages of common types of dies, more particularly, to provide a drawing or extrusion die which has unusually long life and which may be made and shaped with ease to 'a high degree of accuracy so that metal may be drawn thereby to very close tolerances and with extremely smooth finish, which drawing may be either hot or cold.

In accordance with the general principles of the invention, the die holder comprises two parts or halves to form a split holder. The die inserts or nibs are split and are formed from two small blocks of hard material, such as tungsten carbide, which are silver soldered into grooves or wells formed in the holders. The holder halves and nibs are placed in side-by-side relationship and firmly held in such position while a grinding wheel ispassed along the surfaces thereof to effect deep grinding of slots or cavities in the die inserts or nibs to the desired dimension and .fin'al finish, thus eliminating the necessity of hand honing or finishing. Unusually high accuracy in the finished shape of the die is thus obtained since grinding by a grinding wheel, particularly to form the approach angles in the' throat of the die, is much more reliable and accuratethan hand honing. Heretofor'e it was commonly thought by those skilled in the art that deep grinding of tungsten carbide dies was impossible because of the tendice cncy of the surface to crack and ship as a consequence of such grinding. It was further thought that such dies or lnserts have soft cores so that even if it were possible to deep grind them, the cores would have insufficient hardness and resistance to abrasion to provide a suitable die cavity.

I have made the remarkable discovery that, in reality, such cores are harder, rather than softer, than the shells and have found, contrary to common beliefs, that deep grinding, while tending to chip or crack the outside surface, will not cause cracking of the core portion and, in fact, will give such core portion an extremely smooth finish which will enable drawing of metal to very close tolerances. Also, in view of the greater density, hardness and resistance to abrasion of the core portion, it provides a far superior die hole surface than the outer portion and gives a high degree of finish to the drawn metal.

When the halves are held in side-by-side relationship and, either before or after grinding the slots in the nibs, keyways are ground across the surfaces in which the die inserts are seated so that when the halves are afterwards assembled and suitable keys are placed in these keyways, there will be no tendency for lateral displacement of the holder halves as a consequence of the forces developed by the drawing process. Such keying is important for maintaining the die inserts or nibs in proper relationship or regisgry and for assuring accurate shaping of the drawn meta Other objects and advantages of the present invention will become apparent from a study of the following specification taken with the accompanying drawing wherein:

Fig. 1 is a perspective view of a die having a split holder and a split die insert embodying the principles of the present invention,

Fig. 2 is a perspective view of the structure shown in Fig. 1 wherein the die halves and inserts are placed in side-by-side relationship to perm-it grinding of the grooves in the die inserts and of the keyways in both die holder halves;

Fig. 3 is a cross-sectional view taken along line III--III of Fig. 1; and

Fig. 4 is a transverse cross-sectional view of the structure shown in Fig. 2 after the die halves are tilted apart, and illustrating how a grinding wheel G forms the fiat approach angles or tapered throats of the die hole inlet.

Referring more particularly to the drawing, numerals 1 and 2 denote holder halves which form a split holder construction for a die. vA pair of nibs or die inserts 3 and 4 are provided which may be of any suitable harderthan-steel material, preferably tungsten carbide or material having similar hardness. The holder halves may be made of steel. However, when tungsten carbide inserts are used, the holder is preferably made of a material having a relatively low coefiicient of expansion, that is, one approximating that of the tungsten carbide die insert placed therein. A suitable material is an iron-nickelcobalt alloy (Kovar) such as commonly used in glasswtometal seals because of the somewhat matched coefiicient of glass and such alloy. The die insert pair or halves 3 and 4 are fitted into and silver soldered or brazed at 7 in slots or cavities 1a and 221 formed in the holder halves 1 and '2, respectively, and thereby form complementary or half portions which together define a somewhat rectangularly-shaped die hole. The shape of the die hole may be round, rectangular, trapezoidal or any other desired configuration. In the making of copper commutator bars for generators and .motors, the cross section is trapezoidal or wedge-shaped.

The adjoining .faces of the holder halves 1 and 2 are also provided with registering keyways 1b, 10, 2b and 2c into which are fitted suitable keys 5 and 6 which serve to prevent lateral displacement of the die halves and die inserts so as] to insure that the complementary die inserts or halves will be in exact registry even after a considerable number of bars of metal "have been drawn therethrough. I

The assembly shown in Fig. 1 is placed in a common type-box (not shown) having a conventional tapered side to cooperate with one of the inclined sides of the die holder, a wedge being placed between the other inclined side and the box to tightly clamp the holder in place in the box so as to hold it stationary during the drawing process.

An important feature of the invention is the method of making and assembling the die which may be best understood from an inspection of Fig. 2. The die is made by first placing the die halves 1 and 2 in side-by-side relationship, as shown in Fig. 2, the pockets 1a and 2a being previously formed therein by any suitable means. The die inserts 3 and 4 are originally in the form of rectangular blocks with smooth sides. These blocks or nibs 3 and 4 are silver soldered or brazed into place at 7 within the respective wells or cavities 1a or 2a, respectively. The die is held in fixed position while a grinding wheel is passed over the top surfaces of the die holder halves and of the insert halves to effect smoothing thereof. The above-mentioned grinding may be effected by a diamond impregnated grinding wheel. Such Wheel is then used to deep grind the grooves in the tungsten carbide inserts to form the die hole. By suitable choice of grinding wheels, the desired approach angles in the throat of the die may be obtained. A suitable approach angle for cold drawing copper of commutator bars may be of the order of 10 extending from the sides and bottom of the die hole inlet so as to provide an outward flare at the mouth of the die to guide the strip of material as it is fed into the die cavity and to provide gradual reduction in size of the metal being drawn. A small countersunk portion is provided on the opposite end or exit portion of the die hole. The sides of the throat are first made flat by a grinding wheel to provide the proper outward flare and the corners are then rounded off slightly. Inasmuch as all the grinding operations can be done by grinding wheels, thereby eliminating the necessity of hand grinding or honing, it is possible to grind tungsten carbide surfaces to extremely close tolerances and to very exact dimensions. Thus, exactly equal approach angles at the sides, top and bottom of the throat of the die may be obtained as well as extremely fine finish of the die halves which will enable the drawing of long lengths of copper or other metal without twisting of the lengths and will provide an extremely smooth finish on the surface of the drawn metal, eliminating the necessity of any further finishing operations.

While the holder halves are in side-by-side relationship, as shown in Fig. 2, a suitable grinding wheel is employed for cutting out the keyway slots 1b, 2b, 1c and 20, thereby insuring that these slots will be in exact registry when the die holders are assembled in the manner shown in Fig. 1. In order to obtain the desired final finish grinding and approach angles, the holder halves 1 and 2 may be tilted apart about 10 from the vertical by inserting suitable wedges therebetween when in the position shown in Fig. 2 (see Fig. 4) and a grinding wheel G may be passed along the top surfaces thereof in succession to obtain the desired approach angles or taper of the throat (die hole inlet). Of course, the exact design of the die hole is determined by the kind and size of the stock to be drawn, the speed of drawing, and other conditions because for each set of conditions, there is some one best shape for the die holes, and some speed best adapted for the drawing.

In view of the very dense and extremely hard working surface presented by the cores of the tungsten carbide nibs as the result of deep grinding of the nibs, the die inserts will withstand frictional wear from drawing to a remarkable degree and will maintain their shape even after a considerable number of drawings. The die is also capable of withstanding the pull of the copper bar being drawn without bending or breaking. In short, the tungsten carbide split die embodying my invention will have a remarkable long life compared to that of commonly used dies. For example, in cold drawing a wedgeshaped copper bar A x A" x in cross-section, a hard steel die is capable of drawing only about 1,500 pounds of such copper before replacement of the die is necessary. Compared to this, a solid tungsten carbide die is capable of drawing 15,000 pounds before requiring replacement. However, by employing a split tungsten carbide die embodying the present invention, it is possible to draw about 750,000 pounds of copper of the same size before replacement of the die is necessary. Thus, the present novel die has a life at least about fifty times that of the ordinary solid tungsten carbide die. This is extremely important because it eliminates much of the socalled down time or inoperative time of the machine for allowing repairs or replacement of the dies which considerably hampers production and increases the cost of the finished drawn product. Considerably less labor is required for making a die embodying my invention and, in view of the simplicity of construction of the die, the cost thereof is considerably less than that of the conventional die. Thus, a split die embodying the present invention will not only produce as many bars in one setting as older types of dies would in a number of settings, therefore considerably increasing the production rate of drawn bars, but will permit repair or replacement of dies in a matter of hours, instead of days as is generally required for conventional hand honed dies. It has been estimated that the maintenance cost of a hand honed, tungsten carbide die, considering lost time required for repair, is at least twenty times that of the cost of a die embodying the present invention. Even after the dies of the present invention are worn from long use, they may be refinished to coarser size and their life considerably extended.

Thus it will be seen that I have provided an efficient, relatively simple die having remarkable long life and capable of drawing unusually large amounts of metal to extremely close tolerances and with extremely smooth and fine finish and which enables the use of power-driven grinding wheels permitting machine grinding instead of hand grinding or honing, as the result of which accurate dimensions, approach angles, full-aways and the like are obtained which prevents the twisting of the metal as it is drawn through the die and insures accurate final dimensions and extremely small tolerances of drawn metal.

While I have illustrated and described certain specific embodiments of my invention, it will be understood that these are by way of illustration only, and that various changes and modifications may be made within the specification and within the scope of the following claims.

I claim:

1. The method of forming a tungsten carbide draw die having fiat faces, which comprises forming a holder in two halves with a complementary recess in each of said halves, soldering a hardened tungsten carbide insert in each of said recesses, placing said halves in side-by-side abutting relationship with said recesses in coincidence, grinding said inserts simultaneously by a grinding wheel to a depth substantially one-half the depth of the die opening while said halves are positioned in said side-by-side coincidence to form identical grooves therein, and then mounting said halves in confronting and contacting relationship so that the complementary grooves therein cooperate to form a polygonal die opening.

2. The method of forming a tungsten carbide draw die having flat faces, which comprises forming a holder in two halves with a complementary recess in each of said halves, soldering a hardened tungsten carbide insert in each of said recesses, placing said halves in side-by-side abutting relationship with said. recesses in coincidence, grinding said inserts simultaneously to a depth substantially onehalf the depth of the die opening by a grinding wheel and forming rectangular keyways in the faces of said halves simultaneously by a metal removing tool while said halves are positioned in side-by-side coincidence to form identical grooves therein, tilting each of said halves away from each other while in said side-by-side relationship to form an acute angle with respect to a vertical plane therebetween and grinding flat faced approach angles in said inserts by the grinding wheel so as to obtain flat, gradual and uniform outward flare of the die opening and then mounting said halves in confronting and contacting relationship so that the complementary grooves therein cooperate to form a polygonal die opening.

References Cited in the file of this patent UNITED STATES PATENTS Gyuris Dec. 23, 1952 

